1. Field of the Invention
The present invention relates to geophysical seismic exploration wherein an acoustic source is deployed in a wellbore. More particularly, it relates to processing of signals resulting from such an acoustic source.
2. Description of the Prior Art
The use of an energy source positioned within a wellbore to generate a seismic signal for vertical seismic profiling is known. Use of the acoustic signal generated by rotation of the drill bit has also been proposed. In such method, a reference sensor is positioned on the drilling rig. Seismic vibrations from the drill bit are mechanically transmitted through the drill collars and through the drill pipe to the top of the kelly and into the swivel. A reference signal recorded on the rig at the top of the drill string corresponds to the bit generated signal modified by the transfer function between the bit and the reference sensor. The signal detected by the reference sensor is cross-correlated with the signals detected by field sensors to determine the travel times of signals transmitted through the earth and reflected from subsurface interfaces. It is known to flatten the source spectrum and, hence, the cross-correlation function spectrum, by deriving a filter based on the reference signal and convolving that filter response with the cross-correlation of the reference and field signals or with the reference signal prior to cross-correlation. Reference deconvolution causes the resulting wavelet to be sharper, which improves spatial and temporal resolution. It is also known that reference deconvolution attenuates multipath signals, which may be referred to as drill string multiples, in the reference signal. Drill string multiples are signals which have been reflected from locations in the drill string and, therefore, have traveled along at least a portion of the drill string a multiple number of times. However, drill string multiples are still present in the field signals and, consequently, will still be present in the cross-correlation function between the field signal and reference signal. The drill string multiples present in the field signal result primarily from signals reflected from the top of the drill string which then travel back down the drill string and propagate from the lower end thereof into the earth, and signals which are reflected downwardly from the interface between the drill collars and drill pipe and are then propagated into the earth. It is known to remove the long period drill string multiples (those reflected from the top of the drill string) by utilizing a series of cross-correlation functions between the field and reference signals at different borehole depths and applying a multitrace rejection filter around an apparent velocity equal to half the velocity of propagation within the drill string. The short period multiples (those reflected downwardly from the interface between the drill pipe and drill collars), however, will still be present in the field signal. It is an object of this invention to attenuate these short period multiples.